Update TLS configuration to Mozilla's "modern" spec

Details, see:

- https://jve.linuxwall.info/blog/index.php?post/2014/10/09/Automated-configuration-analysis-for-Mozilla-s-TLS-guidelines
- https://wiki.mozilla.org/Security/Server_Side_TLS

Signed-off-by: Alan Orth <alan.orth@gmail.com>
roles/nginx: Update TLS vhost task for Ansible > 1.7.1

Seems there is some YAML sublety that causes this syntax to insert
double spaces on the destination file... using native YAML hashes
are a workaround, see GitHub issues:

https://github.com/ansible/ansible/issues/9067
https://github.com/ansible/ansible/issues/9172

Signed-off-by: Alan Orth <alan.orth@gmail.com>
Downgrade TLS configuration to Mozilla's "intermediate" spec

From looking at the list of clients who would be allowed to connect
when using the "modern" spec, I think I'd be doing more harm than
good to use that config right now...

https://www.ssllabs.com/ssltest/analyze.html?d=alaninkenya.org
https://wiki.mozilla.org/Security/Server_Side_TLS

Signed-off-by: Alan Orth <alan.orth@gmail.com>